In an image projector apparatus such as a microfilm strip reader, printer or combination reader/printer apparatus, the image to be reproduced on the viewing screen is correctly focussed with the projection lens moved with respect to the viewing screen through manipulative efforts of the operator, or viewer, who drives the lens directly or handles the mechanical lens position control means provided in the apparatus.
A problem is encountered in such an image projector apparatus in that the focal point of the projection lens may be disturbed when the microfilm strip (or microfiche) which has been in use in the apparatus or the microfilm holder assembly which has been installed in the apparatus is exchanged with a new one. In order to have a beam of light from the projection lens correctly focussed onto the viewing screen during the reading operation or onto the photosensitive drum during the printing operation, an image projector apparatus is provided with a focus control system. A focus control system for use in a known image projector apparatus having such an automatic focus control system has a focus detect area located externally of the viewing screen so that the path of light from the projection lens to the focus detect area is equal in length to the path of light from the projection lens to the viewing screen. The focal point of the projection lens is thus detected from a beam of light incident on this focus detect area located outside the viewing screen. An example of a focus control system of this nature is disclosed in Japanese Provisional Patent Publication (Kokai) No. 60-227241. A focus control system therein disclosed is however not fully acceptable because it is preferable that the focus detect area located externally of the viewing screen be located in the vicinity of the optical axis of the projection lens, that is, centrally of the viewing screen in consideration of the performance characteristics of the lens.
A focus control system is, basically, operative to detect whether or not a beam of light is correctly focussed onto a projection surface which is in this instance implemented by the viewing screen of an image projector apparatus. Such detection is made upon analysis of the distribution of the intensities of light forming an image focussed onto the projection surface. This means that the focal point at which the beam of light may be focussed onto the projection surface can be detected from a beam of light carrying information including image patterns with any degrees of contrast.
A wide variety of image information is recorded on microfilms and may include such information as to produce no visual image in a central area of the viewing screen. If a focus control system is designed to have a focus detect area in the vicinity of a central area of the viewing screen, the system could not detect the focal point from the beam of light to be incident on the central area of the viewing screen since the beam of light produces no visual image in that particular area of the screen.
In order to solve this problem, the focus detect area of the system may be moved either horizontally or vertically from the central area of the viewing screen. Such location of the focus detect area is however not acceptable for the detection of the focal point of a beam of light carrying two image zones which are spaced apart from each other across a blank area extending either horizontally or vertically of the overall image area which is to result from the beam of light.
The present invention contemplates elimination of such a problem and, accordingly, aims at providing an image projector apparatus having an automatic focus control system which is capable of correctly detecting the focal point of a beam of light passed through a projection lens even if the beam of light may carry image zones spaced apart from each other across a blank area extending horizontally or vertically of the overall image area to result from the beam of light.